We examined the natural hormonal rhythms of the male laboratory rat, and found maturational rhythms (plasma testosterone and LH levels rise at 60 days of age and oscillate until 95 days of age); diurnal rhythms (at 95 days of age, plasma testosterone and LH settle into an 8- hr periodicity, in a normal (12:12) light cycle, in all light or in all dark); and a seasonal rhythm (plasma testosterone and LH surge in February-March). These rhythms constitute the hormonal framework of the male laboratory rat, and constitute a rediscovery of his animality. Our next step was to examine the effect of the investigator upon these rhythms (prehandling increases testosterone, blood collecting depresses it, but LH is not influenced). Finally, we examined the social and sexual effects upon these rhythms (higher testosterone levels are associated (1) with sexually experienced males compared with sexually inexperienced males; (2) with males who mate compared to those who do not; (3) with males who have just mated as compared to those who have not; (4) with cohabitating males compared to males who are living alone; (5) with cohabitating males at certain periods of the female estrous cycle). We plan to continue these studies, broaden them with the measurement of prolactin, and test the following hypotheses: (1) that the 60-day-old surge of testosterone activates the pineal to induce a prolactin diurnal cycle which cues the testosterone diurnal cycle at 95 days of age; (2) that a shifting synergism of prolactin and corticosterone determines the seasonal hormone surge in the spring. Based on our understanding of the laws governing hormonal variability in the male rat, we expect to reevaluate the regulating influences of plasma testosterone and LH levels, and continue our study of the interrelationship of hormones and sexual behavior.